The invention relates electrodeless low-pressure discharge lamp having a discharge vessel which is sealed in a gastight manner and is filled with an ionisable metal vapour and a rare gas, the lamp having a cylindrical core of a magnetic material in which during lamp operation an electromagnetic field is generated in the discharge vessel by means of a metal wire winding surrounding the core and a high-frequency electric power supply unit connected thereto, the magnetic material core being provided with a cooling body. Such a lamp is known from U.S. Pat. No. 4,536,675.
In this known lamp a rod-shaped cooling body of, for example, copper is incorporated in the core of magnetic material (such as ferrite) so as to prevent the temperature of the magnetic core from rising to a too high value during operation. In fact, it has been found that there is a risk of an increase of the specific magnetic losses and a decrease of the magnetic permeability of the material when the core material becomes too hot during operation of the lamp. The light output of the lamp decreases due to the occurring power losses in the core. This phenomenon notably occurs if a relatively high power is applied to the lamp.
It has been found that the magnetic core material is insufficiently cooled by the solid rod in lamps to which a relatively high power is applied.